From cold seawater to hydrothermal fluid flow: thermochronological and geochemical elucidation on the diagenesis of a Guadalupian dolostone reservoir from Sichuan Basin (China)

The origin and evolution of ancient, deeply buried dolostone reservoirs remain elusive, largely due to their complex diagenetic overprinting. This study presents a multidisciplinary investigation using clumped isotope thermometry (Δ₄₇), in-situ U-Pb geochronology, and isotopic and elemental geochemistry to quantitatively reconstruct the diagenetic history of the Guadalupian dolostones, which are important reservoir rocks for hydrocarbon exploration in the Sichuan Basin (southwest China). Initial replacive dolomitization event (Rd1/Rd2) occurred during mid‑late Permian (U-Pb ages = 265–257 Ma) at relatively low temperatures (restored Δ₄₇ temperatures = 7–41 °C), with dolomitizingc fluids being dominantly sourced from Guadalupian and/or slightly younger seawater. During burial, these early-formed, marine dolostones have experienced diagenetic alteration from at least two episodes of hydrothermal fluid flow that are documented in the void‑filling saddle dolomite (SD; Δ₄₇ temperatures = 67–135 °C) and blocky calcite (BC; Δ₄₇ temperatures = 182–189 °C) cements. U‑Pb dating further constrains the timing of the hydrothermal events to 264–234 Ma (SD) and 239–235 Ma (BC), which we interprete to be closely related to two regional tectono-hydrothermal events, i.e., the Emeishan Large Igneous Province and the Indo‑Sinian orogeny. The δ¹⁸O values of parent fluids evolved progressively from near‑marine signatures (Rd1/Rd2) to strongly ¹⁸O‑enriched compositions (up to +17‰ VSMOW for BC), suggesting that the hydrothermal fluids were largely sourced from the deep basin and have been experienced intensive water‑rock interaction with the surrounding rocks. Overall, tectono‑hydrothermal processes, changing these dolostones structurally and geochemically, have improved reservoir quality through: 1) recrystllization of the early marine, tightly-packed dolomites into high-temperature Rd3 dolomites that host considerable intercrystalline porosity, 2) generation of open fractures, and 3) formation of dissolution‑enlarged vugs . This study highlights the critical role of tectonically driven hydrothermal fluid flows in the development of deep-burial dolostone reservoirs. Furthermore, by integrating a robust thermochronological diagenetic framework with well‑defined sedimentary facies and fracture characterization, this approach offers an applicable strategy for predicting reservoir quality in deeply buried carbonate successions within tectonically active sedimentary basins.